1. Field of the Invention
The present invention relates to a method of forming a semiconductor crystal by the beam recrystallization process and a semiconductor device fabricated of a semiconductor crystal formed by such a method.
2. Description of the Related Art
The beam recrystallization process is well known in the art in which the beam energy emitted from a beam energy source such as a laser beam source, an electron beam source, or the like is applied to a thin semiconductor film of silicon for a short period of time to melt the thin semiconductor film in a spot or linear region, and thereafter the melted spot or linear region is solidified into a semiconductor crystal of silicon. The beam recrystallization process is advantageous in that a semiconductor crystal can be formed without heating a base in its entirety to high temperature because the thin semiconductor film is locally melted and solidified in a short period of time. Using the beam recrystallization process, it is possible to form a semiconductor device of silicon crystal on a base such as of glass which is not resistant to heat.
According to the beam recrystallization process, however, the thin semiconductor film is melted in a very short time of about 100 ns. Therefore, only instantaneous crystal growth occurs, resulting in a small semiconductor crystal which is about 0.1 .mu.m across. Attempts to produce a large-diameter semiconductor crystal include (A) a process of varying the intensity of a beam with an optical system, (B) a process of varying the intensity of a beam applied to a specimen with an anti-reflection film or an endothermic film on a surface of the specimen to control a temperature distribution, and (C) a process of varying the dissipation of heat depending on the structure of a specimen to control a temperature distribution. However, these processes fail to produce large-diameter semiconductor crystals stably, and are also problematic in that semiconductor crystals produced by these processes have a variety of diameters. For these reasons, it is impossible to fabricate a semiconductor device whose characteristics are equivalent to those of a semiconductor device fabricated from a single-crystal silicon semiconductor substrate, from a semiconductor crystal that is formed by the beam recrystallization process.
FIGS. 1 and 2 of the accompanying drawings show the manner in which a semiconductor crystal is formed by the conventional beam recrystallization process based on the SOI crystal growth technology which employs no seed. A polysilicon film 112 deposited on an insulation film 110 on a base 100 is formed into an island region of suitable size (see FIG. 1), and then a laser beam or the like is applied to the island region to convert it into a single crystal (see FIG. 2). According to this process, the size, e.g., a width L1 shown in FIGS. 1 and 2, of the island of polysilicon film 112 is virtually the same as the size, e.g., a width L2 shown in FIGS. 1 and 2, of the single-crystal island region 120. The energy applied to the thin semiconductor film by the laser beam is only intensive enough to melt the surface of the thin semiconductor film.
In the conventional process, the polysilicon film 112 is not completely melted. The polysilicon film 112 in the shape of an island is crystallized while its size remains unchanged or is maintained.